A wireless interactive video system disclosed in U.S. Pat. No. 4,591,906, May 27, 1986, by Fernando Morales-Garza, et al. provides for real time interactive digital communication from a large audience of subscribers in urban areas in the vicinity of a central television transmitting station.
The Federal Communications Commission (FCC) has now established in the U.S.A. communication standards for interactive data service allocating wireless transmissions in the 218-219 MHz band for FCC licensing for public use in assigned local central transmitting station (CTS) areas authorizing low power subscriber interaction units of maximum effective radiated power under twenty watts.
Wireless interactive video data service is provided without telephone lines or cable systems over a nationwide network of CTSs in the manner disclosed in U.S. Pat. No. 5,101,267, Mar. 31, 1992, Fernando Morales, by way of satellite transmissions between local area CTSs and a data center with a wireless connection being provided to responding units, or remote transmitter units (RTU) and the local area CTSs.
This nationwide communication capability permits live video programs viewed nationwide, such as world series baseball games, to become interactive for individual subscriber participation. Thus, mass communications over a substantially real time communication system with such large urban area audience participation that would jam any existing public telephone switching network capability are made feasible.
Each local CTS in such a nationwide communication system must be capable of interacting within designated license restrictions in the presence of peak local audience participation without significant switching delays. In so doing, substantially real time interactive two-way connections over a network processing an audience of very large numbers of participants communicating substantially simultaneously can be established.
Prior art two-way radio frequency transmission network technology, as represented for example by portable telephone communication systems, is generally incompatible with efficient substantially real time communication in the presence of heavy subscriber activity. This occurs because telephone systems switching and connection operations must be made compatible with switching instructions from subscriber instruments with coded audio tones at audio frequencies accompanying analog audio messages. Thus with long numeric identification numbers for nationwide long distance connections, typically of ten decimal digits, which must be manually entered while busying lines to complete point-to-point connections as a part of the interconnecting signal data, switching circuits are engaged for very long periods of time inconsistent with substantially real time connections or heavy traffic conditions. Accordingly, busy signals are often encountered thereby restricting the size of a participating audience for immediate connection. As such, necessary re-dialing frustrates the potential using audience. Thus, interactive response that requires telephone exchange communications tends to be delayed and discouraging to participants, and introduces the critical problem of identifying and communicating interactively between subscribers in real time without jammed exchanges and the frustration of encountering busy signals and starting over with a new attempt to communicate.
Similarly, even with the restricted amount of digital data that might be transferred in digital paging system messages, where typically some messages only indicate a short fixed length message such as a calling telephone number, there is little possibility of approaching real time communications in the presence of heavy traffic because of the complexities of the necessary telephone switching networks employed for conveying messages.
In order to process digital information accurately, efficiently, and privately, it is necessary to precisely time and organize the digital data and accompanying commands. For real time two-way digital communications with large audiences wanting prompt access to the message conveyance system or network, synchronous signal timing becomes critical and absolutely necessary for real time interactive communication. In general, audio telephone communications are of an analog nature not critical to timing and are conveyed asynchronously. Thus, prior telephone art signal communication systems are unsuited for adoption in interactive video data systems that convey private point-to-point digital messages on a real time basis for large audiences.
A co-pending patent application Ser. No. 07/966,414, filed Oct. 26, 1992, by G. Dinkins, entitled "Interactive Nationwide Data Service Communication System For Stationary And Mobile Battery Operated Subscriber Units" discloses a two-way interactive communication network. The system disclosed in that application includes a network switching center and provides point-to-point communications between subscriber units at different geographic locations. The network switching center, or hub, is connected via satellite links to a plurality of local CTSs. The local CTSs communicates over an RF link with a number of low-power subscriber units. The low-power subscriber units, are, for example, battery-operated mobile units. The local subscriber units, or remote transmitter units (RTUs), for this system are each located in the vicinity of a CTS. The local RTUs are adapted for synchronous time-division-duplexed receipt and transmission of digital messages. The digital messages are transmitted to and from the local RTUs through different paths.
For transmitting digital messages to a RTU from a local base station, or CTS, the digital messages are transmitted at relatively high power (in the tens of watts range) from a local CTS to the local subscriber unit, or RTUs. For transmitting digital messages from a local RTU to a local CTS, inexpensive, low-power, mobile, small-sized, local subscriber units, or RTUs are utilized. Digital messages are transmitted from these low-power RTUs at relatively low power (in the milliwatt range) to the CTS using an intermediate set of remote receiver units. The low-power subscriber RTU units transmit at milliwatt power levels and the system incorporating such low-power, milliwatt RTUs is called a Milliwatt System.
The intermediate remote receiver units are distributed around a local CTS. The remote receiver receives messages from various low-power subscriber RTUs and communicate with the data processing facility of a CTS using, for example, a wired communication link such as a cable. Messages are compiled as data packets in the data processing facility of the CTS and relayed via satellite links to the network switching center. The Milliwatt System uses time division duplex for the outbound signals transmitted from the local CTS to the local RTUs and for the inbound signals transmitted from the local RTUs to the remote receivers located around the local cell sites. Transmissions in the system alternate between outbound signals and inbound signals.
Additionally, to provide adequate signal coverage, interactive systems must be designed such that two adjacent CTS cell sites have certain common coverage areas with overlapping signal coverage. These coverage-overlap areas between two adjacent CTS cell sites receive signals of approximately equal signal strength from both of the CTS cell transmitters. Because time division multiplexing is used to separate the two transmitting functions, it should be appreciated that the timing between the transmitting and receiving time slots is very important to prevent interference. The interactive video system of co-pending patent application Ser. No. 07/966,414, filed Oct. 26, 1992, by G. Dinkins, entitled "Interactive Nationwide Data Service Communication System For Stationary And Mobile Battery Operated Subscriber Units" has a timing accuracy of 50 milliseconds.+-.20 milliseconds. If both CTSs are allowed to transmit in a relatively random fashion, it is likely that one base cell site may be transmitting on a given frequency while an adjacent CTS is attempting to listen to responses from its local subscriber RTU units on the same frequency. This situation causes interference between the two adjacent systems.
Another copending application Ser. No. 08/257,336, filed Jun. 8, 1994 entitled "GPS Synchronization of CTS Transmitters for an Interactive Network" discloses synchronizing all of the CTSs together using a Global Positioning System (GPS) to provide a very precise time stamp at each CTS. With such a system, accuracy of 1 to 4 microseconds is achieved.
Using the GPS, a precise time signal, or time stamp, is made available locally at each CTS to synchronize a system according to the invention. By synchronizing the start-of-transmission time for all of the CTSs in the system, interference is avoided. The signaling protocol for a system according to the invention used time-division-duplexing. In the Broadcast Mode, an outbound signal sends information from a network switching center, or hub, via a satellite link to a CTS, and then to a RTU. In the Response Mode, an inbound signal sends response information from a RTU to a remote receiver, then to a CTS, and then via a satellite link back to the switching center, or hub.
RTUs located in the region of overlapping coverage, which are located between two CTSs, receive signals of approximately equal strength from each of the CTSs. If each CTS is permitted to randomly start its broadcast transmissions, interference results if one CTS is in the Broadcast Mode and if the another CTS is in the Response Mode attempting to listen to a response from a RTU on the same frequency. The precise time stamp made locally available at each CTS by the GPS system is used to accurately synchronize the various CTSs of an interactive video system.
The CTSs and RTUs operate in a band of carrier frequencies between, for example, 218-219 MHz. Each of the local RTUs is individually identified by reception and transmission of digital address signal pulses in a predetermined timing relationship synchronized with the precision clock signal. The CTSs and RTUs hand-off a communication message for transmission over a path through a single one of the cell subdivision receive-only stations. The RTUs transmit on a plurality of frequencies, and the receive-only intermediate receivers at different subdivision sites operate on different ones of said frequencies within the 218-219 MHz band.
A plurality of CTSs are provided in the network according to the system. The CTS transmission facilities are synchronized with a precision clock signal derived from an independent precision clock source such as a GPS network to provide precision timing for the CTS transmission facilities.
However, all of the aforementioned interactive systems provide such interactivity in conjunction with video data service systems. Consequently, the need has arisen for an interactive radio broadcast data service, and a way of synchronizing an interactive radio broadcast data service to prevent interference between operations in adjacent cell sites.